Fix: Segmentation fault when running several witness nodes on the

same machine. (see #377)

Author: dyakovitsky

libraries/chain/db_getter.cpp

@@ -99,7 +99,14 @@ node_property_object& database::node_properties()
 
 uint32_t database::last_non_undoable_block_num() const
 {
-   return head_block_num() - ( _undo_db.size() - _undo_db.active_sessions());
+   //see https://github.com/bitshares/bitshares-core/issues/377
+   /*
+   There is a case when a value of undo_db.size() is greater then head_block_num(),
+   and as result we get a wrong value for last_non_undoable_block_num.
+   To resolve it we should take into account a number of active_sessions in calculations of
+   last_non_undoable_block_num (active sessions are related to a new block which is under generation).
+   */
+   return head_block_num() - ( _undo_db.size() - _undo_db.active_sessions() );
 }
 
 const account_statistics_object& database::get_account_stats_by_owner( account_id_type owner )const